I'm thinking of offering Machado (0$, keeper for the next year and could keep him for 6+ years at varying contract lengths, in a standard 5x5 league) and Oswald Arcia for Heyward (20$ for 3 years) and Oscar Taveras.

I'm giddy for Taveras and, while I like Machado, I think that package is pretty fair. My question is whether it's at all worth it, or if I'm giving away too much.

Unless I am missing something in how your league is setup, I don't see this deal being accepted. Machado and Taveras are about the same value. Machado will probably be ranked higher this year because he already has a major league job but I think Taveras has as much upside as Machado. This deal is more about long-term value and not just about this year at which point they are pretty much even.

So if those two are even, Arcia isn't close to Heyward's value. Honestly even if the deal was just Machado and Arcia for Heyward, I am not sure it would get done but it would be pretty close. It would then depend on whether the other owner likes Machado and what he thinks of Heyward.